Electron states of YAG probed by energy-loss near-edge spectrometry and ab initio calculations

Al K, Al L-2,L-3, O K, Y L-2,L-3, and Y M-2,M-3 energy-loss near-edge structures (ELNESs) of Y3Al5O12 (YAG) were measured using a dedicated scanning transmission electron microscope equipped with a parallel electron energy-loss spectrometer system. An attempt was made to interpret the experimental edges with the help of calculated near-edge structures. An ab initio orthogonalized linear combination of atomic orbitals method, within the local density approximation (LDA), was used to calculate the electronic structure of YAG. Site-decomposed and symmetry projected local densities of states (LDOS) and photo-absorption cross-sections (PACS) were compared to experimental ELNES spectra. There is reasonable agreement between the five different, measured spectra and calculated near-edge structures. PACS calculations correlated better than LDOS with the experimental edges in terms of relative intensities of the peaks. Through a comparison with site-decomposed LDOS calculations, several features of the experimental Al L and Al K edges were assigned to tetrahedral or octahedral site symmetries in the YAG crystal. The interpretation of the O K edge was more complicated since more than one type of cation was present in the structure. The edge profile correlated with the different oxygen-cation bonds around the oxygen atom, i.e. site symmetry. The best agreement between the calculations and the experimental data was obtained for the yttrium edges.